The performance of dynamic loudspeakers and transducers incorporating moving coil or moving wire in a magnetic field spanning a gap between stationary structures (typically a pole and a yoke) is necessarily limited by the soft magnetic materials (typically iron) used in these stationary structures. Most loudspeakers use a mix of hard and soft magnetic materials to perform the function of guiding a magnetic field from a primary magnet through the gap of the transducer.
In most standard loudspeaker designs, the magnetic flux from a fixed hard magnet or magnets is routed through soft magnetic material used in a pole piece and a yoke. The pole allows the field from the hard magnet to be routed to and focused in the gap between the pole and the yoke where the coil interacts with the field to transduce or convert a varying electrical signal passing through the coil into motion of the coil. The coil is fixed to a diaphragm and movement of the diaphragm produces sound.
The use soft magnetic structures in a loudspeaker creates problems. The varying electrical currents in the transducer coils induce fluctuating magnetic fields in the soft magnetic pole and yoke structures, which can then cause the magnetic field in the gap to fluctuate. This flux modulation causes distortion in the sound produced.